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Types of Selection

Types of Selection

Natural selection influences the frequencies of particular alleles and phenotypes within populations in several different ways. Primarily, natural selection can be directional, stabilizing, or disruptive. Directional selection favors one extreme trait and shifts the population towards that phenotype while selecting against individuals displaying alternate traits. Stabilizing selection favors an intermediate trait with a narrow range of variation. Deviation from the optimal phenotype towards an...

Mutation, Gene Flow, and Genetic Drift

Mutation, Gene Flow, and Genetic Drift

In a population that is not at Hardy-Weinberg equilibrium, the frequency of alleles changes over time. Therefore, any deviations from the five conditions of Hardy-Weinberg equilibrium can alter the genetic variation of a given population. Conditions that change the genetic variability of a population include mutations, natural selection, non-random mating, gene flow, and genetic drift (small population size).

Frequency-dependent Selection

Frequency-dependent Selection

When the fitness of a trait is influenced by how common it is (i.e., its frequency) relative to different traits within a population, this is referred to as frequency-dependent selection. Frequency-dependent selection may occur between species or within a single species. This type of selection can either be positive—with more common phenotypes having higher fitness—or negative, with rarer phenotypes conferring increased fitness.

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Disruptive selection maintains variable pheromone blends in the bark beetle Ips pini.

Alice M Shumate¹, Stephen A Teale, Bruce D Ayres

¹Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA. ashumate@fdu.edu

Environmental Entomology

|January 6, 2012

Summary

This study reveals how disruptive selection maintains genetic variation in bark beetle pheromones. Extreme male pheromone blends increase mating success, driving evolutionary diversity in Ips pini populations.

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Area of Science:

Evolutionary Biology
Chemical Ecology
Behavioral Ecology

Background:

Heritable variation is essential for evolution, yet natural selection often reduces it.
Mechanisms maintaining variation include spatial/temporal fitness variations, frequency-dependent selection, and disruptive selection.
The bark beetle Ips pini uses enantiomeric pheromone blends, offering a model to study variation maintenance.

Purpose of the Study:

To investigate the maintenance of variation in the enantiomeric pheromone blend of the bark beetle Ips pini.
To quantify fitness surfaces for mating success and progeny production in natural populations.
To determine the spatial scales of selection acting on pheromone blends.

Main Methods:

Quantified fitness surfaces for mating and reproduction in natural Ips pini populations.
Assessed the impact of paternal pheromone blend on offspring survival.
Compared spatial scales of pheromone blend variation with larval mortality agent distribution.

Main Results:

Males with extreme pheromone blends achieved significantly higher mating success (up to 1.8 times more mates).
Strong disruptive selection was observed on male pheromone blend, favoring extreme types.
Imperfect assortative mating, combined with disruptive selection, sufficiently maintains variation in this fitness-linked trait.

Conclusions:

Disruptive selection on male pheromone blend, coupled with imperfect assortative mating, maintains heritable variation in Ips pini.
Selection likely operates at the scale of small resource patches, not larger pine stands or individual logs.
This study presents a rare documented instance of diversifying selection in a natural population.